Method of producing a tool steel

ABSTRACT

In a method of producing a tool steel, in particular a hot-work tool steel, a steel of the following analysis: C: 0.25-0.6 weight-%, Si: max 0.15 weight-%, Mn: max. 0.3 weight-%, Mo: 2-5 weight-%, Cr: 0-2 weight-%, W: 1-3 weight-%, V: 0-2 weight-%, Ni: 0-3 weight-%, and a residue of iron and unavoidable impurities due to smelting is smelted and alloyed, wherein a workpiece of this steel is heated up and austenitized at temperatures &gt;Ac 3  and then cooled down, wherein the cooling is effected down to a temperature of 330° C. to 360° C. and the workpiece is held isothermally at this temperature, until the workpiece is completely bainitically transformed, followed by cooling down to room temperature, together with a hot-work tool steel for this purpose and its use.

The invention relates to a method of producing a tool steel and in particular a hot-work tool steel.

The term tool steels describes steel materials used in particular for the working or reshaping of a multiplicity of materials. It includes amongst others cool-work tool steels, plastic mould steels, metal-ceramic steels and hot-work tool steels.

Hot-work tool steels generally describes tool steels which, in use, accept a constant temperature lying in excess of 200° C., while this constant temperature is overlaid by temperature peaks arising from the work cycle. Besides the general mechanical stresses which such steels undergo due to the relevant forming process, they are therefore subject to a further thermal stress. General requirements for hot-work tool steels therefore also include a good so-called fire crack resistance, a type of wear resulting from frequent temperature changes in the surface areas.

In addition, hot-work tool steels must have resistance to the occurrence of heat cracks, which is ensured by a core requirement of hot-work tool steels, namely so-called thermal toughness.

Furthermore, such hot-work tool steels must also of course possess high strength, since they are generally subject to impact compression or tension under heat. Not least of importance are of course good wear properties under heat, in particular a low tendency to adhere relative to the materials to be processed, good resistance to erosion, also to high-temperature corrosion and oxidation, together with good dimensional stability, also in the hot state.

Since these hot-work tools must be mechanically processed, so that they can give other materials the necessary shape under heat, good machinability is likewise a requirement. Especially high stresses occur in particular during high-temperature working of steel materials, in particular if these steel materials are hot when placed in a tool made of a hot-work tool steel and then cooled down in this tool in order to generate hardness.

The properties of steel materials described are determined on the one hand by the chemical analysis of the steel material, but primarily the structure of the hot-work tool steel is critical, in particular for properties such as toughness and strength. In this connection, properties such as toughness and strength, but also thermal conductivity and other important properties, may not be individually enhanced without possible negative effects on other desirable properties. To this extent, hot-work tool steels often require compromises, on the one hand in respect of their chemical analysis, but on the other hand also in terms of their structural formation.

Hot-work tool steels are known for example from CH 165893, in which a chemical analysis is disclosed, also on the other hand a suitable heat treatment for setting certain properties.

Similarly known from AT 144892 is a steel alloy, in particular for hot-work tools and tools or articles which have to a high degree insensitivity to temperature fluctuation, dimensional stability, hot-tensile strength and toughness. A subject of this document are chromium-tungsten-nickel steels, in which the nickel content may be partly or wholly replaced by cobalt, and in which tungsten, cobalt, nickel and chromium may be contained but are preferably chromium-free.

Known from WO 2008/017341 A1 is a method of setting the thermal conductivity of a steel, tool steel, in particular hot-work tool steel and a steel article made from the latter. The steel material according to this document should have much greater thermal conductivity than known tool steels but, with an essentially known steel analysis, gives no indication as to how this higher thermal conductivity may be obtained in practice.

Known from DE 1090245 is the use of a steel for hot-work tools, involving on the one hand a chemical analysis for a hot-work tool steel and on the other hand a heat treatment with cooling down from a temperature in the range of 900° C. to 1200° C. expediently 1000° C. to 1100° C. and tempering treatment at a temperature in the range of 500° C. to 750° C., so that a hardness of 30 to 60 HRC is obtained. In this way, a substantially martensitic structure should be set.

From Maschinenmarkt [Machine Market] 24/2010, pages 58 to 61 it is known that it may be advantageous to induce austempering in steel structures, since the bainite phase combines properties which appear contrary to one another, such as high levels of hardness and toughness. In many applications, though, austempering is not cost-effective owing to the lengthy treatment time. A continuous process of measuring the degree of austempering is intended to optimise this austempering time, while the austempering should in particular avoid residual austenite from the softer structure constituents. It is stated that, in comparison with martensitic hardening, austempering has the outstanding benefit of making possible the simultaneous achievement of very high hardness and high toughness levels. It is true that, through austempering, slightly lower hardness is obtained than that of martensitic structures but toughness, which can be detected as impact energy for example in the notched-bar impact test, is markedly increased. Further benefits of bainitic hardness should be less distortion, good dimensional stability, enhanced resistance to crack growth and the scope for generating residual compressive stress in the layer. It has however emerged that the critical disadvantage of bainitic hardening is the comparatively long holding time in the austempering bath. The bainite transformation is a time-intensive process, with the process time depending on the material structure, the composition of the alloy, the temperatures of austenitizing and of the bainite transformation itself. In principle, austempering here takes place in a three-stage process, in which firstly austenitizing takes place with, as far as possible, complete transformation of the ferrite into austenite. The part is then cooled down so rapidly to the austempering temperature that no ferrite or pearlite occurs. Finally, the austempering temperature is held constant and the transformation from austenite to bainite takes place gradually.

From Design of advanced bainitic steels by optimisation of TTT diagrams and T₀ curves, ISIJ International, Volume 46 (2006), No. 10, pp. 1479 to 1488 it is known, that conventional bainitic steels have a reproducibly good combination of toughness and hardness, but remain behind quench-hardened and tempered martensitic steels. This is attributed to the fact that cementite particles have a worsening effect in the microstructure of the steel, since cementite can act as a defect or crack initiator in fracture-proof steels. The occurrence of cementite during bainite transformation can however be suppressed by a steel composition containing around two weight-% silicon. Thermodynamic and kinetic models are presented so that steels with an optimal bainitic microstructure may be developed; these are comprised of a mixture of bainitic ferrite, plastic-enriched residual austenite and some martensite. Using these models, a set of seven carbides-free bainitic steels with 0.3 weight-% carbon is proposed for production.

Known from DE 600 300 867 T2 is an ingot steel for the production of injection moulds for plastic material or for the production of workpieces for metal processing, which should have a martensitic or martensitic-bainitic structure with a relatively high chromium content.

Known from EP 2 662 460 A1 is a method for the production of steel, in particular as a casting mould or tool, with both a bainitic and also a martensitic phase, wherein the steel should undergo heat treatment including austenitizing followed by rapid cooling, in order to inhibit the formation of stable phases with a transformation temperature above that of the bainite, and to hold the temperature high and long enough to prevent the transformation from austenite to martensite, so that at least 60 percent of the transformation takes place below the martensite start temperature plus 200° C. but above the martensite start temperature minus 50° C., so that at least 70 percent bainitic microstructure with fine carbide-like constituents is obtained and impact toughness in excess of 8 joules is reached, within at least 20 millimetres of the surface of the heat-treated steel. Here the martensite start temperature should be ≤480° C. In particular the silicon content, at 1.3 percent, should be relatively high, as with many steels which are to be austempered.

Known from EP 1 956 100 A1 is a hot-work tool steel and a method for its processing, in which a hot-work tool steel of a given analysis is cooled down to room temperature after solution annealing and then reheated to a temperature above Ac1 and subsequently cooled down to room temperature once more, then subjected to heat treatment, with austenitizing carried out during the first heat treatment.

Known from EP 2 006 398 A1 is a method for the production of a steel material in which a steel material is completely austenitised and then cooled down to the temperature of the pearlite nose of the corresponding steel alloy, where it is held until complete pearlite transformation.

Known from EP 1 887 096 A1 is a hot-work tool steel which is intended to have a considerably higher thermal conductivity than known hot-work tool steels, for which purpose it has a special analysis, which is however practical for any hot-work tool steel

The problem of the invention is to create a method for the production of a hot-work tool steel which ensures bainite transformation in an economically feasible short length of time.

The problem is solved by a method with the features of claim 1.

A further problem is to create a hot-work tool steel with which the method may be implemented.

The problem is solved by a method with the features of claim 7.

The austempering according to the invention of the steel material is brought about by providing that, after austenitizing treatment of the steel, it is cooled down to s holding temperature and is held at this holding temperature until austempering is completed.

Here the steel material according to the invention makes it possible for the austempering holding time to lie in roughly the same time range as the austenitizing holding time, which means that austempering is made possible in a time which is absolutely economical. In this connection the holding time depends in particular on material strength and material quantity respectively, i.e. in particular the time within which the necessary holding temperatures occur.

According to the invention this austempering may be measured in a dilatometer and, for the respective steel material, the relevant period of time depending on material strength may be determined. The reduction in temperature following austenitizing is also accompanied by shrinkage due to reduced thermal expansion wherein, immediately after the holding temperature has been reached a relative change in length occurs due to the formation of the bainite. Once the relative change in length is concluded, the possible bainite transformation is completed wherein, as explained above, a complete bainite transformation may be achieved with the steel material according to the invention in contrast to customary hot-work tool steels. Cooling down to room temperature then takes place, leading to further shrinkage, wherein however as compared with the material before bainite transformation, the material after bainite transformation does not return to the value before transformation, but remains somewhat above it.

A steel with the following composition has proved to be advantageous:

-   -   C: 0.25-0.6 weight-%     -   Si: max 0.15 weight-%     -   Mn: max. 0.3 weight-%     -   Mo: 2-5 weight-%     -   Cr: 0-2 weight-%     -   W: 1-3 weight-%     -   V: 0-2 weight-%     -   Ni: 0-3 weight-%

Residue of Iron and Impurities Due to Smelting

Within this analysis it is possible, with an isothermal holding step between 330° C. and 360° C., to bring about a suitable complete bainite transformation.

Temperatures below this temperature window effect martensite formation; higher temperatures lead to formation of the upper intermediate stage, which has poorer mechanical properties.

The invention is explained below by way of example with the aid of a drawing showing in:

FIG. 1 the measurement of the change in length over the temperature programme in a dilatometer test of a steel according to the invention;

FIG. 2 a representation of the change in length relative to temperature during heating up and cooling down including the isothermal holding time;

FIG. 3 the course of the change in length relative to temperature during heating up and cooling down after the isothermal transformation;

FIG. 4 the micro-section of the completely austempered but not tempered steel material at an isothermal holding temperature of 330° C.;

FIG. 5 the micro-section of a steel material according to the invention, which shows complete transformation, austempered at 360° C. and not tempered;

FIG. 6 the course of the relative change in length and temperature during a dilatometer test with a conventional hot-work tool steel;

FIG. 7 the course of the relative change in length depending on the temperature during heating up and cooling down including an isothermal holding step, for a conventional hot-work tool steel;

FIG. 8 the course of the relative change in length and temperature during the dilatometer test at a holding temperature of 340° C., for a conventional hot-work tool steel;

FIG. 9 the course of the relative change in length depending on the temperature during heating up and cooling down including an isothermal holding at 340° C., for a conventional hot-work tool steel;

FIG. 10 the bainite content depending on holding temperature and holding time for a conventional hot-work tool steel, showing that a complete bainite transformation is not possible;

FIG. 11 hardness and notch-bend impact energy for a steel according to the invention which has undergone bainite transformation according to the invention, dependent on tempering temperature;

FIG. 12 temperature conductivity and thermal conductivity of a steel which has been heat-treated according to the invention and a steel which has not been austempered according to the invention;

FIG. 13 the residual austenite content depending on the cooling rate for a steel according to the invention which is not heat-treated according to the invention;

FIG. 14 the micro-section, the relative change in length and the hardness of a steel according to the invention quenched at λ=0.08, without complete bainite transformation;

FIG. 15 structure, relative change in length and hardness after quenching at λ=1.1;

FIG. 16 structure, relative change in length and hardness after quenching at λ=3;

FIG. 17 time and temperature pattern for cooling at λ=1.1 and λ=3;

FIG. 18 an example of a heat treatment curve.

The steel according to the invention for implementing the method according to the invention with the result of a complete bainitic structure (if not otherwise stated, all percentage details are weight-%) has the following composition:

-   -   C: 0.25-0.6 weight-%     -   Si: max 0.15 weight-%     -   Mn: max. 0.3 weight-%     -   Mo: 2-5 weight-%     -   Cr: 0-2 weight-%     -   W: 1-3 weight-%     -   V: 0-2 weight-%     -   Ni: 0-3 weight-%

The remainder is iron and impurities due to smelting.

Such a steel is smelted and alloyed in the usual manner for hot-work tool steels.

In the heat treatment according to the invention (FIG. 18) such a steel is firstly austenitized at a temperature which lies at least above the austenitizing temperature (Ac₃) and ensures a complete full austenitizing of the material or workpiece. For this purpose a certain holding time may be necessary, depending on the workpiece and its dimensions. After austenitizing, which is conducted at heating rates of 300° C./h to 400° C./h, rapid cooling down is effected at cooling rates of λ=0.4 to λ=2, in particular to temperatures between 330° C. and 360° C. This temperature is held, with the holding time similarly depending on the workpiece geometry.

In particular the alloying position determines the cooling rate. Irrespective of the cooling rate, the cooling must be carried out with sufficient speed that no bainite transformation takes place during cooling.

After the bainite transformation is concluded in the holding phase, the workpiece may be cooled down to room temperature, wherein the cooling rate here lies between λ=0.3 and λ=1. As a result, a complete bainitic structure is then available, with residual austenite of less than 1 weight-%.

An example of heat treatment is revealed by FIG. 18, wherein the specimen has dimensions of 370 mm×150 mm×60 mm.

Here the broken line indicates the required furnace value or furnace temperature and the solid line shows the temperature development of the test specimen material. It can be seen that, during a first heating-up to 650° C., the material follows and with a holding time of four hours, the furnace required temperature is also reached by the charge actual value. There then follows a further heating stage, which includes a rise of approx. 200° C./h and lasts for around two hours. After around one and a half hours, the material here also reaches the required temperature value and is then heated at a heating rate of approx. 260° C./h to the austenitizing temperature of over 1100° C. This temperature is reached relatively quickly by the material.

From this heat treatment, the cooling down and holding at a predetermined temperature is then effected, as is evident for example from FIG. 1.

Within the specified steel analysis it is possible to obtain, with the heat treatment process according to the invention, a complete bainitic structure within reasonable heat treatment times, including heating up, quenching and holding time.

Here it is noteworthy that the holding times for austempering lie substantially in the range of holding times for austenitizing, which was previously unachievable in any circumstances.

Surprisingly a temperature range was found in which the special material according to the invention may be transformed completely into bainite in a technically reasonable time and if necessary adjusted in terms of strength by a tempering process.

In contrast to conventional materials, in the case of the material according to the invention, no residual austenite which transforms into martensite remains after the heat treatment according to the invention.

The invention is explained further with the aid of examples.

EXAMPLE 1 (ACCORDING TO THE INVENTION)

A material according to the invention with the following analysis:

C 0.31 weight-% Si 0.10 weight-% Mn 0.24 weight-% P <0.005 weight-% S 0.0004 weight-% Cr 0.05 weight-% Mo 3.22 weight-% Ni 1.95 weight-% V <0.005 weight-% W 1.74 weight-% Ti <0.005 weight-% Al 0.01 weight-% is smelted and alloyed in the customary manner. The material is brought into the form of a dilatometer specimen, as a cylinder with a diameter of 5 mm and a length of 10 mm. This is used for the dilatometer tests.

A further test specimen in the form of a notched-bar specimen measuring 55 mm×10 mm×10 mm is heated to austenitizing temperature, with the austenitizing set at 1090° C. The test specimen is held at this temperature for 15 min. and then cooled down to 330° C. Here the rate of cooling is around 40° C. per second.

The test specimens are held isothermally for 17 min. and then cooled down to room temperature.

The resultant change in length of the dilatometer test specimen over the temperature range is shown in FIG. 1. There the temperature rise on the one hand and the relative change in length on the other hand are shown as percentages, wherein it is evident that, after the rapid cooling down from the austenitizing temperature to 330° C., a relative extension takes place, which approaches a maximum which is held. With the subsequent cooling down there is an irreversible lengthening even at room temperature as compared with the very small extension after reaching the austempering temperature. In FIG. 2, the relative change in length is plotted as a percentage against temperature, wherein it may be seen that, on completion of cooling down from the austenitizing temperature to the austempering temperature with isothermal holding, a change in length results, so that a hysteresis loop occurs, in particular between heating up and cooling down.

After the isothermal holding time the relative change in length runs in percentage terms proportionally to the cooling down (FIG. 3).

FIG. 4 shows the micro-section of the dilatometer test specimen, with the Vickers hardness amounting to 494, while the Rockwell hardness (HRC) comes to 50.5. A complete transformation of the material into bainite may be seen from the micro-section. The residual austenite content is <1% and is therefore insignificant for the material properties.

EXAMPLE 2 (ACCORDING TO THE INVENTION)

The material of example 1 is smelted and alloyed in a comparable manner and then subjected to comparable heat treatment, wherein however the cooling down from the austenitizing temperature of 1090° C. is to 360° C., with the cooling rate coinciding with that of example 1.

The micro-section, made after cooling down to room temperature, is shown in FIG. 5. Here too the residual austenite content is >1%, while Vickers hardness is 494 and Rockwell hardness (HRC) comes to 47. Here too a complete transformation is obtained.

EXAMPLE 3 (NOT ACCORDING TO THE INVENTION)

A conventional hot-work tool steel with a chemical composition of

-   -   C=0.38 weight-%     -   Si=0.10 weight-%     -   Mn=0.40 weight-%     -   Cr=5.00 weight-%     -   Mo=1.30 weight-%     -   V=0.40 weight-%         conforming to DIN EN as material 1,2343 or X 38 Cr Mo V5-1 is,         after smelting and alloying, heated to an austenitizing         temperature of 1030° C. and held at that temperature until         austenitizing is completed. The material is then cooled down         quickly to a holding temperature of 320° C. where it is held         until change in length is constant and then cooled down to room         temperature.

FIG. 6 shows the pattern of the relative change in length and temperature during the dilatometer test.

The pattern of the relative change in length depending on temperature during heating and including the isothermal holding time as described is evident from FIG. 7, wherein it is clear that there is no closed hysteresis.

EXAMPLE 4 (NOT ACCORDING TO THE INVENTION)

The material according to example 3 is austenitized in the same manner at 1030° C., but cooled down to an isothermal holding temperature of 340° C. The pattern of the relative change in length depending on temperature during heating and cooling down including the isothermal holding time is here shown in FIGS. 8 and 9, while here too it is evident that, although a certain lengthening due to bainite formation does take place, the lengthening then reduces and in particular a closed hysteresis curve is not obtained.

The bainite contents of example 3 (holding temperature=320° C.) and example 4 (holding temperature=340° C.) with the respective holding times in hours are shown in FIG. 10.

From this it is evident that, in examples 3 and 4, maximum austempering is achieved only after 5 hours, which is many times the maximum achievable austempering of the material according to the invention, obtainable within a maximum of 20 min. Moreover, with a holding temperature of 340° C. a maximum austempering of around 55% is obtained, whereas with a holding temperature of 320° C. a maximum austempering of 80% is achieved. The remaining structure is here similarly in the form of residual austenite or martensite.

In the case of tempering after heat treatment, for example 1 a pattern of notch-bend impact energy and hardness respectively corresponding to FIG. 11 may be observed. Depending on the tempering temperature for two hours tempering time in each case, the Rockwell hardness may be varied between 47 and 52, while in each case notch-bend impact energy lies between 8 joules at room temperature (i.e. in the untempered state) and 5 to 6 joules, indicating very even hardness and toughness.

EXAMPLE 5 (NOT ACCORDING TO THE INVENTION)

The material according to the invention is subjected to a conventional heat treatment on the dilatometer, with the test specimen dimensions corresponding to those of the previous examples.

Here, differing λ-values are used, with λ in this case being the cooling parameter, which is usual in the heat treatment of tool steels. It indicates in hectoseconds the time needed for cooling down a steel from 800° C. to 500° C.

Hardness and residual austenite patterns for different λ-values are evident from FIG. 13 wherein, with slow cooling, hardness as expected falls from around 550 Vickers hardness to 325 Vickers hardness, whereas residual austenite content increases with rising λ-values.

With λ=0.08 and a conventional heat treatment, the material according to the invention has a martensitic structure, with the structure pattern shown in FIG. 14, similarly the relative change in length and temperature during cooling.

With λ=1.1 the structure is characterised by martensite and an intermediate stage in which the change in length adopts a different pattern.

In FIG. 16 the structure is specified for λ=3, with martensite and an intermediate stage and a basically different change in length to temperature relationship.

FIG. 17 shows, for cubes with edge lengths 75 mm and 180 mm, how temperature and time behave, wherein cooling of the smaller cube is effected with λ=1.1, while cooling of the larger cube is effected with λ=3.

The tests show impressively the success in finding a combination of a chemical composition of the steel on the one hand and a heat treatment on the other hand, which makes possible a completely bainitic structure of a hot-work tool.

The material according to the invention discloses this potential only with the heat treatment according to the invention; a conventional heat treatment does not lead to the desired result.

Conversely, the heat treatment according to the invention similarly does not produce the result with materials which are not according to the invention.

In the case of the invention it is advantageous that, in a reproducible and reliable way, it is possible to produce a tool steel which has a bainitic structure and therefore outstanding tough-hard properties, which may be controlled by suitable tempering and in accordance with specific hardness properties, with altogether high notch-bend impact energy. 

1. Method of producing a tool steel, in particular a hot-work tool steel, wherein a steel of the following analysis C: 0.25-0.6 weight-% Si: max 0.15 weight-% Mn: max. 0.3 weight-% Mo: 2-5 weight-% Cr: 0-2 weight-% W: 1-3 weight-% V: 0-2 weight-% Ni: 0-3 weight-% and a residue of iron and unavoidable impurities due to smelting is smelted and alloyed, wherein a workpiece of this steel is heated up and austenitized at temperatures >Ac₃ and then cooled down, wherein the cooling is effected down to a temperature of 330° C. to 360° C. and the workpiece is held isothermally at this temperature, until the workpiece is completely bainitically transformed, followed by cooling down to room temperature.
 2. Method according to claim 1, wherein after complete through heating for a period of 20 to 40 min., austenitizing takes place.
 3. Method according to claim 1, wherein isothermal holding takes place for a period of 30 to 90 min.
 4. Method according to claim 1, wherein cooling from the austenitizing temperature down to the isothermal holding temperature is conducted at cooling rates from λ=0.4 to λ=2.
 5. Method according to claim 1, wherein after cooling to room temperature, the workpiece is tempered to set the hardness.
 6. Method according to claim 1, wherein the tempering temperature lies between 580° C. and 660° C., while the duration of tempering lies between 1 and 2 hours.
 7. Hot-work tool steel produced by a method according to claim 1 and with an analysis as follows: C: 0.25-0.6 weight-% Si: max 0.15 weight-% Mn: max. 0.3 weight-% Mo: 2-5 weight-% Cr: 0-2 weight-% W: 1-3 weight-% V: 0-2 weight-% Ni: 0-3 weight-% and a residue of iron and unavoidable impurities due to smelting, wherein the hot-work tool steel has a residual austenite content <1% with an otherwise completely bainitic structure, wherein hardness lies between 46 and 53 HRC and notch-bend impact energy 4.5 and 8.5 joules.
 8. Use of a tool steel according to claim 7 for hot-work tools such as injection moulds, hot-working tools, press hardening tools, forging dies, hammer dies, hot-piercing punches, hot-extrusion punches. 